Advancements in micromachining and other microfabrication techniques and processes have enabled the fabrication of a wide variety of MicroElectroMechanical Systems (MEMS) and devices. These include moving rotors, gears, switches, accelerometers, miniaturized sensors, actuator systems, and other such structures.
One popular approach for forming MEMS devices makes use of a modified wafer known as a Silicon-On-Insulator (SOI) wafer. An SOI wafer is essentially a silicon wafer having a silicon dioxide sacrificial layer disposed thereon, and having a film of active single-crystalline silicon disposed on the sacrificial layer.
MEMS devices fabricated on SOI wafers have a number of advantages. The formation of a MEMS device on such a wafer occurs in single crystal silicon that is of very high mechanical quality. Consequently, the components of the device can be made with high thickness and have inherently low mechanical stress. Moreover, because the components of the device are fabricated from single crystal silicon, the device can be readily integrated with CMOS devices and other such structures.
In the processing of a MEMS device, it is often necessary to make electrical contact to the handle wafer which provides support for the MEMS structures. One method of doing so involves the use of a thin epitaxial layer to selectively etch away the device layer and sacrificial layer. The device silicon can then be grown to the required thickness on the exposed substrate. However, this approach typically results in a non-planar surface as a result of defect propagation during epitaxial growth. A non-planar surface is undesirable because the fine line lithography commonly used to develop surface features on the device has limited depth focus.
Another problem encountered during the fabrication and use of MEMS devices relates to stiction. Stiction refers to the phenomenon in which a moving component of a MEMS device adheres to an adjacent surface. Stiction typically occurs when surface adhesion forces between the component and the adjacent surface are higher than the mechanical restoring force of the micro-structure. These surface adhesion forces may arise from capillary forces, electrostatic attraction, or direct chemical bonding. In MEMS devices such as accelerometers, it is imperative that the fingers of the device are not subject to vertical stiction, since this can cause the device to malfunction. Unfortunately, stiction becomes increasingly problematic as device sizes are reduced, and hence it has become a greater obstacle to overcome as MEMS devices have become more sensitive.
One approach to preventing stiction is through the formation of anti-stiction protrusions on SOI wafers. A known method for making anti-stiction protrusions involves a wafer comprising a silicon substrate, a silicon oxide sacrificial layer and a silicon device layer. A series of trenches are etched in the device silicon to expose the sacrificial layer. The sacrificial layer is then laterally etched with hydrofluoric acid (which does not etch silicon) until one or more thin portions of the sacrificial layer remain. Then, the device silicon and silicon substrate are isotropically etched with a solution of KOH. Since the KOH does not attack the material of the sacrificial layer, the remaining portion of the sacrificial layer acts as a mask to the silicon underneath it. Hence, protrusions are formed where the remaining portion of the sacrificial layer attaches to the device layer and to the substrate. The etch of the sacrificial layer may then be completed, leaving behind a series of protrusions on the opposing surfaces of the substrate and the device layer. While this method can be reasonably effective at forming anti-stiction protrusions, process variations in the wet etch steps frequently result in inconsistent protrusion or device thickness, and consequent variations in device performance.
There is thus a need in the art for a method for producing a MEMS structure on a substrate, and particularly on an SOI wafer, that allows for a high degree of surface planarity on the wafer after electrical contact to the handle wafer has been made. There is also a need in the art for a method for producing anti-stiction protrusions in a MEMS structure that achieves a consistent device thickness. These and other needs are met by the methodologies and devices disclosed herein and hereinafter described.